Multi-stage cryogenic engine



P 3, 1954 s. F. MALAKER ETAL 3,147,600

' MULTI-STAGE CRYOGENIC ENGINE Filed June 19, 1963 2 Sheets-Sheet 1 -l6 l3- IE V e 9 a FIG. I

INVENTORS STEPHEN F. MALAKER JOHN G. DAUNT ATTORNEY Sept. 8, 1964 s. F. MALAKER ET AL 3,147,600

MULTI-STAGE CRYOGENIC ENGINE Filed June 19, 1963 2 Sheets-Sheet 2 I N VENTORS STEPHEN F. MALAKER JOHN G- DAUNT ATTORNEY United States Patent Jersey Filed June 19, 1963, Ser. No. 28%067 4 Claims. (Cl. 626) This invention deals with a multi-stage cryogenic engine for providing refrigeration to extremely low temperatures. More specifically, it deals with a refrigeration engine operating on a modified Stirling cycle having two or more expansion volumes and employing a working gas as an expansion medium whereby rotary mechanical power put into the engine is used for the extraction of heat from the low temperature parts of the engine.

In US. Patent 3,074,244, issued on January 22, 1963, to Stephen F. Malaker and John G. Daunt, there is described a cryogenic engine operating on a modified Stirling cycle and capable of cooling to low temperatures, in the range down to 50 K., when driven by an electric motor or similar rotating motive power. Since the thermodynamic relations of the Stirling cycle itself and of the modified Stirling cycle used in said miniature cryogenic engine, and the motor driven engine employing this modified cycle have been adequately outlined in aforesaid patent, no attempt will be made here to discuss the theoretical aspects involved. The matters that are discussed here concern a novel development of said cryogenic engine which has led to unexpected and satisfactory results for producing refrigeration to temperatures well below 50 K. using a modified Stirling cycle refrigerating engine which em ploys two or more expansion volumes connected in a novel manner. There are many present-day requirements for refrigeration well below 50 K. for the cooling of electronic devices, for the liquefaction of very low temperature boiling liquids and for many other applications in the fields of electronics, cryogenics and space technology. As has been outlined in aforesaid patent, cryogenic engines operating on a modified Stirling cycle answer these multiple requirements in a most satisfactory manner, provided they can reach to sufficiently low temperatures. The aforementioned patented cryogenic engine operating in a single stage can provide cooling only down to temperatures of about 50 K. To obtain cooling at lower temperatures, using modified Stirling cycles, one procedure would be to cascade two or more of such cryogenic engines. The first and largest of such cascaded engines would operate from ambient temperature, which temperature environment would provide the heat sink for the engine, and, say, 90 K. which would be the low temperature head of said first engine in the cascade. The second engine of the cascade, which would be smaller than the first, would operate between the temperature of the head of said first cascaded engine and a still lower temperature, say, 30 K. The head of said first cascaded engine would provide the heat sink for the second said cascaded engine and the head of said second cascaded engine would handle the refrigerative load at the lower temperature of, say, 30 K. If necessary, further cascading could be made, the third and possible subsequent cascaded engines being connected in the same manner as the connection between the first and second said cascaded engines. A possible embodiment of such an adaptation of cascading cryogenic engines of the type covered in aforesaid patent is illustrated schematically in FIGURE 1 which shows partly in crosssection and partly cutaway a perspective side-elevational view of such a refrigeration system. FIGURE 2 presents a side elevational view, partlycut away and partly in crosssection, of a preferred embodiment of a two-stage cascaded system made according to the present invention.

3,147,600 Patented Sept. 8, 1964 Referring to the drawing in FIGURE 1, numeral 2 refers to a gas-tight crank case containing the working gas, say, helium, under high pressure of, say 250 p.s.i.a. which encloses crankshaft 3 driven by electric motor 4, said crankshaft having four cranks which, through respective connecting rods, reciprocate pistons 5, 6, 7 and 8, which reciprocate in suitable cylinders arranged parallel to one another and at right angles to said crankshaft 3. Piston 5 and its associated cylinder forms first compression volume 9 of the first stage engine of the two-stage cascaded refrigeration system. Piston 6 which has insulating extension 10 extending vertically therefrom, together with its associated cylinder, forms first expander volume 11 of the first-stage engine of the two-stage refrigeration unit. The phase relation of the motions of pistons 5 and 6 is that appropriate for modified Stirling cycle and determined by making the angle between the cranks which serve to cause the motion of said pistons approximately The upper portion of the engine is covered with a housing 64 containing insulating material 65. The working gas is constrained by the motion of pistons 5 and 6 to move alternately between first compression volume 9 and first expander volume 11, which gas passes through first heat exchanger 12, first regenerator 13 and first heat-transfer conduit 14, which said heat-transfer conduit is thermally-attached to first cold head 15, the cold head being made of good thermally-conductive material. The second cascaded engine is composed of compressor piston 7 and expander piston 8 both having thermally-insulating extensions 16 and 17 respectively attached to them, which pistons move also in approximately 90 phase relationship to each other. Compressor piston 7 and its extension 16 together with its cylinder form second compression space 18. Expander piston 8 and its extension 17 together with its cylinder form expansion space 19. Working gas alternately passes between second compressor space 18 and second expander space 19 and in this alternating passage working gas passes through second heat exchanger 20, second regenerator 21 and second heat-transfer conduit 22. Heat-transfer conduit 22 is formed as part of second cold head 23 at which the desired refrigeration is performed. It is to be noted, as shown in FIGURE 1, that second heat exchanger 20 is a part of and in intimate thermal relation to first cold head 15, so that the heat of compression produced in second compression space 18 is transferred from the working gas passing from second compression space 18 to second expansion space 19 to the material of second heat exchanger 20 and thence to the material of first cold head 15, said heat therefore forming part of the thermal load on first cold head 15 whereby working gas passing from second compression space 18 through second heat exchanger 20 is cooled to the temperature of first cold head 15. It is evident, therefore, that this method of cascading of modified Stirling refrigeration engines of the type described in aforesaid patent is one in which essentially the first cascaded cooling engine and the second cascaded cooling engine are pneumatically separate, the thermal connection between the cold head of the first stage and the compression volume of the second stage being made through second heat exchanger 20 without there being any interconnection of the Working gas of one engine with the working gas of the other. Further engines could be cascaded for reaching to still lower temperatures in the same manner as that illustrated in FIGURE 1.

This invention deals with an unexpectedly superior method of cascading, one superior thermodynamically and mechanically, and which can be more readily understood by reference to FIGURE 2, in which a preferred embodiment of a two-stage cascaded system is illustrated, the drawing being a side elevational view, partly cutaway and partly in cross-section.

The basic features of the invention are that only two pistons are used, a compressor piston and an expander piston, the latter providing for two or more expansion volumes and the fact that the same working gas passes between the one compressor volume and all expander volumes. Since the expander volumes are produced by the same one-multiple piston, the variations of their volumes are identical in phase; this fact together with the fact that only one working gas is used to permeate all expansion volumes assures the correct phase relationship between all expanders and the single compressor volume.

Numeral 24 refers to a gas-tight crank case containing the working gas, say, helium, which encloses crankshaft 25 and driving shaft 26, whereby crankshaft 25 is rotated through the action of drive motor 27. The crankshaft carries compressor crank 28 and expander crank 29 arranged to have a crank angle between them of approximately 90". Compressor crank 23 has connected to it connecting rod 59 which serves to reciprocate compressor piston 30 in compressor cylinder 31. Expander crank 28 has connected to it connecting rod 32 which Serves to reciprocate expander piston 33 in expander cylinder 34. Compressor piston 30 and expander piston 33 carry rings (for simplicity not shown in the diagram of FIG. 2) which serve to prevent blow-by of gas through the space between each piston and its respective cylinder wall. Expander piston 33 carries an extension made of two parts, 35 and 36, such that the upper part 36 is of smaller diameter than the lower part 35, which, in turn, is slightly smaller in diameter than piston 33. This stepped extension (35 and 36) is made of material of poor thermal conductivity, say, ceramic or laminated phenolformaldehyde plastic, to minimize unwanted heat flow between the warm parts of the engine and the refrigerated cold parts. Expansion cylinder 34 is extended in an upward direction by having a thin-walled cylinder 37 of the same diameter and coaxial therewith, said extension 37 terminating in first cold head 38. The purpose of making the upper part 37 of said expander cylinder of thin-walled material of poor thermal conductivity (such as stainless steel, for example), is to minimize unwanted heat flow between the warm end 34 of the expander cylinder and the cold head 38. The first expander space 39 is formed between the first step of expander piston sections 35 and first cold head 38 (made of copper, for example). Extending vertically upward from first cold head 38 is a thin-walled tube 40 made of material of poor thermal conductivity, and arranged so that tube 40 is coaxial with expander cylinder 34 and expander cylinder extension 37. Within said tube 40 reciprocates the second, narrower diameter expander piston extension 36. Said tube 40 terminates at the top end in second cold head 41. The space between the top end of said second extension 36 of expander piston 33 and second cold head 41 is second expansion volume 42.

The motion of compressor piston 30 and expander piston 33 causes the working gas alternately to move from compressor volume 43, formed by the top of compressor piston 30 and cylinder head 44, and expansion volumes 42 and 39. On contraction of compressor volume 43 the working gas first passes through channel heat exchanger 45, so transferring the heat of compression to the walls of heat exchanger 45, thence to the metallic material of cylinder head 44 from which it is finally dissipated to the ambient surroundings through cooling fins 66 across which an appropriate cooling fluid is permitted to flow. The working gas issuing from heat exchanger 45 passes to first regenerator 46, the purpose and construction of which for the operation of modified Stirling cycle refrigerating engines has been adequately described in aforesaid patent. The gas issuing from regenerator 46 is fed through conduits 47 and 48, conduit 48 being part of cold head 33, and from conduit 48 the gas flow is subdivided into two flows, one passing into expansion volume 39 through heat transfer channels 49, which serve to permit heat transfer between the expanded working gas and cold head 33. The

other path simultaneously taken by the working gas passing through conduit 48 passes through conduit 50 into second regenerator 51, which is of design smaller than said regenerator 46. On issuing from regenerator 51, the working gas passes through conduits 52 and 53, conduit 53 being part of second cold head 41. From conduit 53 the working gas passes into second expansion volume 42, through heat transfer channels 54, which serve to permit heat transfer between the gas expanded in second expansion volume 42 and second cold head 41. At each alternate part of the cycle the reverse process of flow takes place in which the working gas flows from first expansion volume 39 and second expansion volume 42, when they are being reduced in volume, back into compressor volume 43, via said heat transfer conduits, channels, regenerators, and heat exchanger. Attached to cylinder head 44 is vacuum jacket 55 which can be evacuated through valved nipple 56 and which serves to insulate thermally the upper end 37 of said expansion cylinder, regenerators 46 and 51, cold heads 38 and 41, conduits 47, 5t) and 52. and heat transfer channels 49 and 54 and the upper eXtnsions 35 and 36 of expander piston 33 from ambient temperature surroundings. Attached to first cold head 38 1 is radiation shield 57, made of good thermally conducting material which surrounds second cold head 41, regenera tor 51, conduit 52 and tube 40 and which maintains itself at the same temperature as cold head 33.

In the operation of said preferred embodiment shown in FIGURE 2, the alternate displacement of the gas from compressor volume 43 into expander volumes 39 and 42 and vice versa, results in a cooling of cold head 38 and radiation shield 57 to a temperature which can be K., and cooling of cold head 41 to a temperature which can be 30 K. It is to be noted that in this embodiment described herewith the expansions and contractions of the gas in expansion volumes 39 and 42 are in phase with each other, and both bear the same phase relationship to the contraction and expansion of the working gas in com pressor volume 43, the phase relation being that required for the operation of a modified Stirling cycle as described in aforementioned patent.

There are many important thermodynamic and mechanical advantages in the invention over other cascading methods, an example of which latter has been illus trated in FIGURE 1, and described earlier. The first is the striking reduction in the number of moving parts in the invention, as compared with the engine illustrated in FIGURE 1. By reduction in the number of moving parts a marked increase in the reliability is attained, and since, in general, such refrigerators are required to operate un attended for thousands of hours, any important feature leading to increased reliability is of extreme value. It is to be noted that this question of reduction of the number of moving parts becomes even more significant and more outstanding in the case where an engine should be made that has more than two stages. In the cascading unit illustrated in FIGURE 1 the number of moving parts is almost exactly proportional to the number of stages.

For a multiple cascaded refrigeration system according to the present invention, further stages can be added by adding further reduced stepped extensions to the single expansion piston and, associated with each stepped extension, the appropriate non-moving components, viz. a regenerator, suitable conduits, a cold head, and a heat-transfer conduit. It is apparent, therefore, that a refrigerative system, according to the present invention, would have the same number of moving parts regardless of the number of stages that are cascaded, since the multiple stepped expander piston constitutes one moving part only. In addition, the number of fixed components in a cascaded refrigerating system, according to the present invention, also will be less than the number required in a cascaded system, according to the design illustrated in FIGURE 1. It is to be noted, for example, in the two-stage system illustrated in FIGURE 1, that there must be two heat interchangers, a first interchanger 12 and a second heat interchanger 20. In the present invention, as shown in the embodiment illustrated in FIGURE 2, since there is only one compressor volume (43), there need be only one heat exchanger (45). This is an important improvement in that not only is the total number of components reduced, but also the number of small passages, which could possibly get blocked by contaminations, also is reduced, thus leading to further gains in reliability.

One of the important thermodynamic advantages of this invention lies in the fact that any gas which might leak out of the second expansion volume by blow-by past the reciprocating extension of the expander piston operating on said second expander volume and the cylinder wall or extension of the cylinder wall surrounding said extension will be transmitted directly into the first expansion volume. Such unwanted wastage of refrigerative power occasioned by said possible blow-by is less serious in a device according to our invention than it would be in a device according to FIGURE 1. In the device made according to this invention, such blow-by transmits gas from the lowest temperature level to the next lowest temperature level, whereas in the device according to FIG- URE 1 said possible blow-by transmits the gas from the lowest temperature level to ambient temperature. The net loss in the second case, therefore, is greater and, because of such blow-by, the device according to FIGURE 1 is thermodynamically less efiicient. Another source of thermodynamic inefiiciency lies in unwanted heat leak through the material parts of the device from the high temperature parts to those parts at which it is desired to maintain low temperatures. In the embodiment of this invention, illustrated in FIGURE 2, it will be seen that such conductive heat leaks to the low temperature parts are much less than in the devices illustrated in FIGURE 1. The heat leak to the first cold head of the device of FIGURE 2 can only be through expander cylinder extension 37, which is thin-walled, or the body of expander piston extension 35, which is of material of poor thermal conductivity, or through the metal parts of conduit 47, the casing or housing of first regenerator 46, or the metal parts of conduit 58, which connect regenerator 46 to cylinder head 44, which cylinder head is at ambient temperature. In the device of FIGURE 1 the first cold head is connected to the ambient temperature by at least twice as many paths as the device of FIGURE 2, occasioned by the duplication of compressors and expanders. Such a reduction in possible losess due to unwanted thermal conductivity in the device, according to this invention, makes it highly desirable thermodynamically, and permits it to operate with unexpectedly high efficiency. For one watt load at 30 K. the two-stage system of the present invention has the following approximate Moreover, the system for cascading modified Stirling cycle cryogenic engines, according to this invention, in which a stepped expander piston is used to provide a multiplicity of expansion volumes in which only one compressor volume is employed in which the number of moving parts is independent of the number of stages of cascading, results in a cryogenic system of great reliability, me-

chanical simplicity, high thermodynamic efiiciency and,

being a self-contained unit requiring no external compressors or gas supplies, comprises a unit of considerable logistic convenience. Furthermore, the system, according to this invention, readily lends itself to any number of cascades, as has been delineated above.

The engine of the present invention may be used to cool devices or materials of all types. For example, as shown in FIGURE 2, it may be used to cool an infra red detector 70 which is thermally bonded to the top surface of cold head 41. The terminals of cell 70 are connected to wires '71 and 72 which are led through hole 73 in radiation shield 57 and connected to plug outlet or connector 74 mounted in sealed relation into the wall of vacuum jacket 55. To permit performance of cell 70, a hole 75 is cut in the top of radiation shield 57. Also, an infra red transmitting window 76 is mounted in the top of vacuum jacket 55, and sealed in vacuum-tight relation therewith at its edge 77.

We claim:

1. In a multi-stage cryogenic engine for providing refrigeration at low temperatures and operating with parallel pistons on a modified Stirling cycle, wherein a motor drives a crankshaft rotating cranks to which are attached connecting rods disposed at approximately with respect to each other, and operating on a working gas under pressure in a sealed crank case, the improvement comprising,

a compressor piston connected to a connecting rod, and

a compressor cylinder in which said piston is designed to reciprocate, said cylinder terminating in a head,

an expander piston connected to another connecting rod, and an expander cylinder in which said latter piston is designed to reciprocate,

a gas-permeable heat exchanger disposed above said compressor cylinder head and designed to transfer compression heat from compressed gas to a cooling means,

an expander piston extension of low thermal conductivity attached to said expander piston in axial relation therewith and extending beyond the expander cylinder, said piston extension being stepped so that it is of two diiferent diameters, the lower part thereof having approximately the diameter of said expander piston and the upper part thereof having. a smaller diameter,

a first extension of said expander cylinder comprising a tube of low thermal conductivity connected at one end to the top of said expander cylinder and surrounding at least a portion of the lower part of said expander piston extension,

a first cold head of high thermal conductivity connected to the other end of said tube in a manner such as to form a first expander volume between said first cold head and the top of the said lower part of said expander piston extension, and having a conduit therein designed to permit flow of working gas into and out of said first expander volume and to permit heat transfer between working gas and said first cold head, and having a central opening designed to permit the said upper part of said expander piston extension to reciprocate therethrough,

a first regenerator disposed above said gas permeable heat exchanger and having one end connected therewith to allow Working gas therefrom to flow therethrough and the other end connected with said conduit in said first cold head for flow of gas therethrough and designed alternatingly to absorb heat and reject heat from working gas flowing alternately therethrough,

a second extension of said expander cylinder comprising a tube of low thermal conductivity, connected at one end to said first cold head and surrounding the upper part of said expander piston extension,

a second cold head of high thermal conductivity serving as the coldest part of said engine and connecting to the other end of said second extension of said expander cylinder in a maner such as to form a second expander volume between said second cold head and the top of said upper portion of said expander piston extension, and having a conduit therein designed to permit flow of Working gas into and out of said second expander volume and to permit heat transfer between working gas and said second cold head,

a second regenerator disposed above said first cold head and having one end connected pneumatically to the junction between the said first regenerator and said conduit in said first cold head, and the other end connected with said conduit in said second cold head, whereby working gas is permitted to flow through said second regenerator and into and out of said second expander volume, and

an insulating means designed to insulate thermally all parts of the engine which are maintained at temperatures below those of the compressor and expander cylinders.

2. A multi-stage cryogenic engine according to claim 1 in which the expander piston extension is stepped so as to permit more than two expander volumes, said latter expander volumes being maintained at successively lower temperatures, and which engine has the appropriate and necessary numbers of expansion cylinder extensions, cold References Cited in the file of this patent UNITED STATES PATENTS 2,966,034 Gifford Dec. 27, 1960 2,966,035 Gifford Dec. 27, 1960 3,074,244 Malaker et a1. Jan. 22, 1963 

1. IN A MULTI-STAGE CRYOGENIC ENGINE FOR PROVIDING REFRIGERATION AT LOW TEMPERATURES AND OPERATING WITH PARALLEL PISTONS ON A MODIFIED STIRLING CYCLE, WHEREIN A MOTOR DRIVES A CRANKSHAFT ROTATING CRANKS TO WHICH ARE ATTACHED CONNECTING RODS DISPOSED AT APPROXIMATELY 90* WITH RESPECT TO EACH OTHER, AND OPERATING ON A WORKING GAS UNDER PRESSURE IN A SEALED CRANK CASE, THE IMPROVEMENT COMPRISING, A COMPRESSOR PISTON CONNECTED TO A CONNECTING ROD, AND A COMPRESSOR CYLINDER IN WHICH SAID PISTON IS DESIGNED TO RECIPROCATE, SAID CYLINDER TERMINATING IN A HEAD, AN EXPANDER PISTON CONNECTED TO ANOTHER CONNECTING ROD, AND AN EXPANDER CYLINDER IN WHICH SAID LATTER PISTON IS DESIGNED TO RECIPROCATE, A GAS-PERMEABLE HEAT EXCHANGER DISPOSED ABOVE SAID COMPRESSOR CYLINDER HEAD AND DESIGNED TO TRANSFER COMPRESSION HEAT FROM COMPRESSED GAS TO A COOLING MEANS, AN EXPANDER PISTON EXTENSION OF LOW THERMAL CONDUCTIVITY ATTACHED TO SAID EXPANDER PISTON IN AXIAL RELATION THEREWITH AND EXTENDING BEYOND THE EXPANDER CYLINDER, SAID PISTON EXTENSION BEING STEPPED SO THAT IT IS OF TWO DIFFERENT DIAMETERS, THE LOWER PART THEREOF HAVING APPROXIMATELY THE DIAMETER OF SAID EXPANDER PISTON AND THE UPPER PART THEREOF HAVING A SMALLER DIAMETER, A FIRST EXTENSION OF SAID EXPANDER CYLINDER COMPRISING A TUBE OF LOW THERMAL CONDUCTIVITY CONNECTED AT ONE END TO THE TOP OF SAID EXPANDER CYLINDER AND SURROUNDING AT LEAST A PORTION OF THE LOWER PART OF SAID EXPANDER PISTON EXTENSION, A FIRST COLD HEAT OF HIGH THERMAL CONDUCTIVITY CONNECTED TO THE OTHER END OF SAID TUBE IN A MANNER SUCH AS TO FORM A FIRST EXPANDER VOLUME BETWEEN SAID FIRST COLD HEAD AND THE TOP OF THE SAID LOWER PART OF SAID EXPANDER PISTON EXTENSION, AND HAVING A CONDUIT THEREIN DESIGNED TO PERMIT FLOW OF WORKING GAS INTO AND OUT OF SAID FIRST EXPANDER VOLUME AND TO PERMIT HEAT TRANSFER BETWEEN WORKING GAS AND SAID FIRST COLD HEAD, AND HAVING A CENTRAL OPENING DESIGNED TO PERMIT THE SAID UPPER PART OF SAID EXPANDER PISTON EXTENSION TO RECIPROCATE THERETHROUGH, A FIRST REGENERATOR DISPOSED ABOVE SAID GAS PERMEABLE HEAT EXCHANGER AND HAVING ONE END CONNECTED THEREWITH TO ALLOW WORKING GAS THEREFROM TO FLOW THERETHROUGH AND THE OTHER END CONNECTED WITH SAID CONDUIT IN SAID FIRST COLD HEAD FOR FLOW OF GAS THERETHROUGH AND DESIGNED ALTERNATINGLY TO ABSORB HEAT AND REJECT HEAT FROM WORKING GAS FLOWING ALTERNATELY THERETHROUGH, A SECOND EXTENSION OF SAID EXPANDER CYLINDER COMPRISING A TUBE OF LOW THERMAL CONDUCTIVITY, CONNECTED AT ONE END TO SAID FIRST COLD HEAD AND SURROUNDING THE UPPER PART OF SAID EXPANDER PISTON EXTENSION, A SECOND COLD HEAD OF HIGH THERMAL CONDUCTIVITY SERVING AS THE COLDEST PART OF SAID ENGINE AND CONNECTING TO THE OTHER END OF SAID SECOND EXTENSION OF SAID EXPANDER CYLINDER IN A MANER SUCH AS TO FORM A SECOND EXPANDER VOLUME BETWEEN SAID SECOND COLD HEAD AND THE TOP OF SAID UPPER PORTION OF SAID EXPANDER PISTON EXTENSION, AND HAVING A CONDUIT THEREIN DESIGNED TO PERMIT FLOW OF WORKING GAS INTO AND OUT OF SAID SECOND EXPANDER VOLUME AND TO PERMIT HEAT TRANSFER BETWEEN WORKING GAS AND SAID SECOND COLD HEAD, A SECOND REGENERATOR DISPOSED ABOVE SAID FIRST COLD HEAD AND HAVING ONE END CONNECTED PNEUMATICALLY TO THE JUNCTION BETWEEN THE SAID FIRST REGENERATOR AND SAID CONDUIT IN SAID FIRST COLD HEAD, AND THE OTHER END CONNECTED WITH SAID CONDUIT IN SAID SECOND COLD HEAD, WHEREBY WORKING GAS IS PERMITTED TO FLOW THROUGH SAID SECOND REGENERATOR AND INTO AND OUT OF SAID SECOND EXPANDER VOLUME, AND AN INSULATING MEANS DESIGNED TO INSULATE THERMALLY ALL PARTS OF THE ENGINE WHICH ARE MAINTAINED AT TEMPERATURES BELOW THOSE OF THE COMPRESSOR AND EXPANDER CYLINDERS. 